The present invention relates to a method for manufacturing photovoltaic devices having semiconductor bodies comprised of amorphous silicon.
Photovoltaic devices, such as solar cells, are capable of converting solar radiant energy into usable electric energy. The energy conversion occurs as a result of what is known as the "photovoltaic effect". An amorphous silicon solar cell is comprised of a body of hydrogenated amorphous silicon (a-Si:H) material, which is typically formed in a glow discharge of silane. Such cells are of the type described in U.S. Pat. No. 4,064,521 entitled SEMICONDUCTOR DEVICE HAVING A BODY OF AMORPHOUS SILICON which issued to D. E. Carlson on Dec. 20, 1977 and which is herein incorporated by reference.
Within the body of the cell there is an electric field which results from the different conductivity types of the semiconductor regions comprising the body.
Typically, a P-I-N structure is used, and when light impinges upon the body photons generate electron-hole pairs in the intrinsic region of the body. In an amorphous silicon solar cell, the drift mechanism which results from the inherent electric field within the body causes electrons to flow toward the N type region and holes to flow toward the P type region which are on opposed sides of the intrinsic region. Thus, if there is an external circuit connecting the N type region to the P type region, current will flow through that circuit as long as light continues to generate electron-hole pairs in the solar cell.
As is now well known by those skilled in the art, amorphous silicon solar cells are typically fabricated by the glow discharge of silane (SiH.sub.4). The process of glow discharge involves the discharge of energy through a gas at relatively low pressure in a partially evacuated chamber. In particular, the glow discharge of silane is typically conducted at a pressure not greater than about 5 Torr. As is described more fully in U.S. Pat. No. 4,142,195 entitled SCHOTIKY BARRIER SEMICONDUCTOR DEVICE AND METHOD OF MAKING SAME which issued to D. E. Carlson et al. on Feb. 27, 1979, a typical process for fabricating an amorphous silicon solar cell comprises placing a substrate on a heated element within a vacuum chamber. A screen electrode, or grid, is connected to one terminal of a power supply, and a second electrode is connected to the other terminal of the power supply such that the screen electrode is between the second electrode and the substrate. While silane, at low pressure, is admitted into the vacuum chamber, a glow discharge is established between the two electrodes and an amorphous silicon film deposits upon the substrate.
The amorphous silicon P-I-N structure can be formed either in the manner described in U.S. Pat. No. 4,064,521, in which case the substrate is typically comprised of a metal, such as aluminum, niobium, tantalum, chromium, iron, bismuth, antimony, or stainless steel. In a typical process, the amorphous silicon is doped by adding impurities to the silane. For example, the first dopant may be diborane (B.sub.2 H.sub.6), which is added to the silane to form a P type amorphous silicon layer. After the P type layer has been formed to a thickness on the order of one hundred Angstroms, the diborane flow is turned off to form an intrinsic region having a thickness on the order of a few thousand Angstroms. Thereafter, an N type dopant, such as phosphine (PH.sub.3), is added to the silane flow in order to form an N type amorphous silicon layer having a thickness of a few hundred Angstroms. On the N type layer, a transparent, conductive layer is formed. In a typical process, indium tin oxide (ITO) is used for this purpose.
As is well known by those skilled in the art, the P and the N type regions may be reversed. Similarly, the substrate may be formed of glass rather than a metal. In such event, a transparent, conductive coating, such as ITO, is applied to the glass substrate prior to forming the amorphous silicon. Thereafter, the cell can be formed either P-I-N or N-I-P with a metallic contact on the back, i.e., the surface removed from the substrate. These various cell types are described in the last mentioned patent to Carlson and in U.S. Pat. No. 4,162,505 issued to J. J. Hanak.
While numerous techniques have been described for forming amorphous silicon solar cells, the present state of the art is such that techniques are desired for increasing the efficiency and fill factors of such cells.